Retainer
Also known as: maintenance plan, care plan
A fixed monthly fee for ongoing attention to a website, rather than a price for one specific job.
A retainer is the client paying you to keep caring about their site. Updates, small changes, watching the numbers and telling them what to do next.
Good retainers describe what the client receives. Weak ones describe how many hours you will spend, which invites an argument about whether you spent them.
The honest test of a retainer is whether you would be comfortable if the client asked, in month seven, what the last six payments bought. If the answer is a feeling rather than a document, the retainer is at risk.